OBJECTIVES: To study the characteristics of a release of peptides from tumor cells, which we have recently observed as an injury by alloantibody, and properties of the peptides released by this reaction, especially size and degree of completion of the molecules. To study intracellular mechanisms of the peptide release itself: the mechanism by which an antibody against a cell membrane component could be terminating synthesis of a protein within the cell. To seek a more sensitive form of anti-tumor antibody detection. Thus far we have had available, except for the much more laborious colony inhibition, only anti-tumor cytotoxicity. In contrast, peptide release is a measure of injury involving synthetic functions of the cell, and may well be a more sensitive indicator. To seek a more sensitive system for measuring cell-mediated immunity, both allo and anti-tumor. Again, this would use the greater sensitivity of a synthesis-related phenomenon and the possibility of diminishing the cell numbers in the reaction mixtures, within reliable ranges of radioactive counts.